doc: add two tips for speeding the dev builds

Add two important tips for novice Node.js contributors

PR-URL: #36452
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: mmomtchev

BUILDING.md

@@ -30,6 +30,7 @@ file a new issue.
     * [Building the documentation](#building-the-documentation)
     * [Building a debug build](#building-a-debug-build)
     * [Building an ASAN build](#building-an-asan-build)
+    * [Speeding up frequent rebuilds when developing](#speeding-up-frequent-rebuilds-when-developing)
     * [Troubleshooting Unix and macOS builds](#troubleshooting-unix-and-macos-builds)
   * [Windows](#windows)
     * [Prerequisites](#prerequisites)
@@ -527,6 +528,29 @@ $  ./configure --debug --enable-asan && make -j4
 $ make test-only
 ```
 
+#### Speeding up frequent rebuilds when developing
+
+If you plan to frequently rebuild Node.js, especially if using several branches,
+installing `ccache` can help to greatly reduce build times. Set up with:
+```console
+$ sudo apt install ccache   # for Debian/Ubuntu, included in most Linux distros
+$ ccache -o cache_dir=<tmp_dir>
+$ ccache -o max_size=5.0G
+$ export CC="ccache gcc"    # add to your .profile
+$ export CXX="ccache g++"   # add to your .profile
+```
+This will allow for near-instantaneous rebuilds even when switching branches.
+
+When modifying only the JS layer in `lib`, it is possible to externally load it
+without modifying the executable:
+```console
+$ ./configure --node-builtin-modules-path $(pwd)
+```
+The resulting binary won't include any JS files and will try to load them from
+the specified directory. The JS debugger of Visual Studio Code supports this
+configuration since the November 2020 version and allows for setting
+breakpoints.
+
 #### Troubleshooting Unix and macOS builds
 
 Stale builds can sometimes result in `file not found` errors while building.